Cognitive radio is known, which recognizes a radio environment in its vicinity and optimizes communication parameters depending on the radio environment. However, when a frequency band allocated to a wireless communication system is shared by another wireless communication system, it is necessary to consider an issue of interference. Hereinafter, a system on the side originally allocated a frequency band and suffering interference will be referred to as a primary system (interfered system), a system on the side secondarily using this frequency band and causing interference will be referred to as a secondary system (interfering system), a receiving station of the primary system will be referred to as a primary receiving station, and a transmitting station of the secondary system will be referred to as a secondary transmitting station.
When the secondary system shares a frequency band with the primary system, the secondary system needs to avoid affecting existing services provided by the primary system. To this end, the transmit power of secondary transmitting stations needs to be adjusted such as to be not greater than maximum transmit power, at which primary receiving stations can maintain predetermined reception quality. Hereinafter, this maximum transmit power will be referred to as acceptable transmit power, and actual transmit power adjusted to be not greater than the maximum transmit power will be referred to as actual transmit power. Here, conceivable criteria for maintaining the predetermined reception quality include: maintaining the CIR (Carrier to Interference Ratio) or CINR (Carrier to Interference plus Noise Ratio) of a primary receiving station at a predetermined value or higher; and keeping the amount of interference suffered by a primary receiving station at a predetermined value or lower.
For example, PTL 1 discloses a system for controlling the transmit power of secondary transmitting stations such as not to interfere on a primary receiving station. According to PTL 1, a value is calculated by dividing interference power acceptable to a primary receiving station (hereinafter, referred to as acceptable interference power) by the number of secondary transmitting stations, and acceptable transmit power is calculated such that interference from a secondary transmitting station will be not greater than that value. Thus, gross interference from the plurality of secondary transmitting stations is suppressed to the acceptable interference power or lower, whereby the primary receiving station can maintain predetermined reception quality even when the plurality of secondary transmitting stations perform transmission at the same time.
Hereinafter, an outline of a system architecture as assumed in PTL 1 will be described with reference to FIG. 1. Referring to FIG. 1, spectrum managers SM (coordinators in PTL 1) manage use of frequencies by secondary transmitting stations Ts. More particularly, the spectrum manager SM1 manages the secondary transmitting stations Ts11 and Ts12, and the spectrum manager SM2 manages the secondary transmitting stations Ts21 and Ts22. Moreover, the spectrum managers SM1 and SM2 share information related to the secondary transmitting stations Ts under their respective own management through a common database DB (a management node in PTL 1). According to PTL 1, disclosed are two processes for determining acceptable transmit power, namely, centralized-type, and autonomous distributed-type.
In the centralized-type, information on a plurality of secondary transmitting stations Ts is aggregated at a database DB, and the database DB calculates acceptable transmit power for all the secondary transmitting stations Ts (Ts11, Ts12, Ts21, and Ts22). The calculated acceptable transmit power is notified, through the spectrum managers SM, to each secondary transmitting station Ts under their respective own management.
In the autonomous distributed-type, the spectrum managers SM share information related to the secondary transmitting stations Ts (e.g., the total number of the secondary transmitting stations) under their respective own management through a database DB. Each spectrum manager (e.g., SM1) calculates acceptable transmit power for the secondary transmitting stations under its own management (e.g., Ts11 and Ts12), taking into consideration the interference that the secondary transmitting stations (e.g., Ts21 and Ts22) under management of the other spectrum manager (e.g., SM2) may cause to the primary receiving station.